1. Field of the Invention
This invention relates to electrical switching apparatus and, more particularly, to circuit breakers employing a trip unit. The invention also relates to circuit breaker trip units.
2. Background Information
Circuit breakers and circuit breaker trip units are well known in the art. See, for example, U.S. Pat. Nos. 5,910,760; 6,144,271; and 6,850,135.
As disclosed in U.S. Pat. No. 6,144,271, a plunger of a trip unit is employed to trip open an associated circuit breaker frame whenever the plunger is extended from the trip unit. Actuation of primary and secondary frame latches occurs exclusively by way of the extended and resettable trip unit plunger, which is, otherwise, normally contained entirely within the trip unit. The secondary frame latch is in disposition to be struck by an abutment surface of the extended plunger. In response to a reset operation, the trip unit is also reset whenever the secondary frame latch drives the extended plunger in the opposite direction against its plunger spring and into the trip unit.
It is known to provide a fuse having an indicator or plunger for indicating a triggered or open fuse condition. See, for example, U.S. Pat. Nos. 3,783,428; 4,766,408; 5,319,344; 5,886,613; and 6,256,183.
U.S. Pat. No. 5,426,406 discloses a molded case circuit breaker unit including an accessory compartment within the circuit breaker cover. A field-replaceable fuse is contained within the compartment for protecting an electric motor without tripping upon motor current reversal. An electronic trip unit within the circuit breaker is adjusted for short time over-current protection, while the fuse is selected to protect against short circuits. The fuse can optionally be installed within the circuit breaker accessory enclosure or in a separate compartment attached to the circuit breaker housing. Upon the occurrence of a short circuit within the protected circuit, the fuse operates to isolate the protected equipment and is conveniently replaced without disassembling the circuit breaker components.
U.S. Pat. No. 5,587,570 discloses a molded case circuit breaker including a fuse enclosure and an interlock unit. Upon the occurrence of a short circuit, one or more indicating fuses operate to isolate the protected equipment. Interference between a tab of the interlock unit and the indicators of the fuses prevents the operating handle from being moved to a reset position to turn the circuit breaker on. When the fuses are replaced, the normal positions of the indicators allow the movement of the operating handle from the tripped position to the reset position.
U.S. Pat. No. 5,835,002 discloses an interlock assembly for use in a manually operated multi-phase fusible switch having a fuse in series with a blade for each phase and a handle for simultaneously controlling the position of the blades.
U.S. Pat. No. 6,710,988 discloses a molded case electric switch housing including an electric switch control circuit for motor overload and phase loss conditions, a motor contactor control circuit for turning an associated electric motor on and off, and a replaceable fuse unit for handling short-circuit type faults.
There is a need for an improved electrical switching apparatus employing a trip unit.
There is also a need for an improved trip unit.